1. Field of the Invention
The present invention relates to an improved method for producing microcapsules and, more particularly, to a method for producing microcapsules in which the wall is formed from a formaldehyde condensation product. While the method of the present invention is useful in producing microcapsules generally, microcapsules prepared in accordance with the present invention are particularly useful in photosensitive imaging materials of the type which employ a layer of microcapsules containing a radiation sensitive composition.
2. Brief Description of the Prior Art
Imaging materials employing photosensitive microcapsules are the subject of commonly assigned U.S. Pat. Nos. 4,399,209 and 4,440,836.
In the aforesaid commonly assigned U.S. patents, images are formed by image-wise exposing a layer of photosensitive capsules to actinic radiation and rupturing the capsules typically by passing the imaging sheet containing the capsules through a pressure nip. The radiation sensitive composition contains a photohardenable or photosoftenable material which undergoes a change in viscosity or modulus upon exposure. For example, in the most typical embodiments, the radiation sensitive composition contains a polyethylenically unsaturated monomer which polymerizes upon exposure, thereby causing the phase internal to the capsules to become harder. Due to the difference in the hardness of the capsules in the exposed versus the unexposed areas, only certain capsules rupture and release their contents. If the internal phase contains a dye precursor, the precursor is image-wise released, and a color image is formed upon its transfer to a developer layer. In previously disclosed embodiments, the developer layer may be present on the same support as the layer of capsules or a separate support.
As a microcapsule wall material, amine-formaldehyde polymers, and more particularly, melamine-formaldehyde polymers have been used to enwrap the photosensitive internal phase. For example, U.S. Pat. No. 4,608,330 discloses the encapsulation of photosensitive materials of the types described above in melamine-formaldehyde capsules. The process for preparing the microcapsules typically involves forming an emulsion of an oily internal phase composition in an aqueous phase, and enwrapping particles of the internal phase in a formaldehyde condensation product produced by in situ condensation of melamine and formaldehyde. This patent also discloses that a polyisocyanate may be added to the oily internal phase composition. As is described in greater detail in the patent, the presence of the isocyanate functions to form a prewall around the internal phase upon introduction into the aqueous phase as a result of the interfacial polycondensation polymerization reaction occurring between the isocyanate groups of the oily phase and the hydroxy groups of the aqueous phase.
Similarly, U.S. Pat. No. 4,353,809 discloses microcapsules with melamine-formaldehyde or ureaformaldehyde capsule shell walls. The reference also teaches the addition of a polyvalent isocyanate, a polyvalent isothiocyanate or a prepolymer thereof to the internal phase prior to addition to an emulsion to form a prewall material around the internal phase. The presence of the isocyanate, isothiocyanate, or prepolymer functions to stabilize the emulsion and enable the production of small diameter microcapsules.
Although the above microencapsulation processes have proven to be of great importance, they still suffer from one or more minor drawbacks.
For example, the prewall polycondensation reaction between the isocyanate groups and the hydroxy groups takes a relatively long time, and any reaction which occurs is confined to the oil-water interface. Due to the long prewall formation time period, the overall production of the microcapsules takes a relatively long time, and is cost inefficient. When trying to produce the microcapsules on a commercial scale, the cost expenditures associated with requiring the prewall materials to react to completion results in prohibitive costs.
When the internal phase of the microcapsules contains a photosensitive composition, other problems must be considered. In microcapsule imaging systems of the types described above, the photosensitive composition is extremely oxygen sensitive in that photopolymerization can only take place in its absence. Accordingly, the microcapsule walls must effectively functions as a barrier material to reduce permeation by oxygen. Further, the microcapsule walls should be as water resistant as possible. While the presence of water per se does not harm the photosensitive properties of the internal phase, water can plasticize the wall material and reduce the wall's barrier properties. As a result, if the barrier properties are broken down, the amount of oxygen diffusion will increase and, in turn, the photographic efficiency of the internal phase will significantly decrease. The above described prior art microcapsules, although able to generally form an effective barrier against oxygen and/or water, do not form a complete barrier.
A further problem associated with the microcapsules having a prewall formed by addition of a polyisocyanate to the aqueous phase is the potential presence of excess unpolymerized isocyanato groups. When polyisocyanates are added to an aqueous phase during emulsification, the polyisocyanates react with water to form a prewall at the oil-water interface. The product, a crosslinked polyurea polymer, is insoluble in both water and polyisocyanate, and the degree of swelling decreases as the crosslinking density increases. The polyurea polymer may eventually behave as a barrier that separates the two reactants and as such, prevents any further reaction from occurring after a layer of crosslinked prewall has been formed at the interface. Accordingly, under normal conditions, it is expected that some unreacted isocyanates will be present in the internal phase of the capsules. Upon exposure to moisture, the unreacted isocyanates will be converted to amines which may impede the efficiency of free radical photoinitiation, especially when compounds such as the cyanine dye/borate anion initiators as disclosed in U.S. Pat. No. 4,772,530 are used. In the cases where time, temperature, humidity and the concentration of unreacted amines are favorable to the formation of a network inside the capsules, a loss in image density may result.
Accordingly, there exists a need in the art for photosensitive microcapsules wherein the microcapsule walls are capable of forming rapidly, act as excellent oxygen and water barriers, are capable of providing an extended shelf life and do not interfere with the imaging chemistry of the internal phase.